In liquid crystal displays (LCDs) it is usually required to control the alignment of the liquid crystal medium at the inner surface of the substrates forming the display cell. For example, parallel or tilted orientation of the liquid crystal molecules relative to the plane of the substrate is achieved by applying rubbed polyimide alignment layers to the substrate surfaces. Another common method to induce uniform alignment is for example the oblique evaporation of inorganic materials like silicon-oxide (SiOx) onto the substrate surfaces.
Reviews of conventional alignment techniques are given for example by I. Sage in “Thermotropic Liquid Crystals”, edited by G. W. Gray, John Wiley & Sons, 1987, pages 75–77, and by T. Uchida and H. Seki in “Liquid Crystals—Applications and Uses Vol. 3”, edited by B. Bahadur, World Scientific Publishing, Singapore 1992, pages 1–63. A review of alignment materials and techniques is given by J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981), pages 1–77.
Many applications, like for example LCDs of the VA (vertical aligned) or SSCT (surface stabilized cholesteric texture) mode, require vertical or so-called homeotropic alignment of the liquid crystal medium, wherein the liquid crystal molecules are oriented with their long molecular axis substantially perpendicular to the plane of the substrate. In prior art, the following techniques have been suggested to achieve homeotropic alignment:
1. Use materials that intrinsically have very low surface energies, for example fluorinated polymers such as PTFE. In this case the energy of the system is minimized by having the LC molecules in contact with each other rather than with the surface. This leads to a homeotropic alignment, but with rather a weak anchoring energy.
2. Coat the substrate surface with a surfactant that “seeds” the required alignment. For homeotropic alignment one can achieve this by using a layer of hydrocarbon chains tethered at one end to the surface. Just from steric considerations one expects that if the surface coverage is sufficient these chains will pack to be on average normal to the surface. If the interaction between the chains and the LC molecules is sufficiently strong then this alignment should seed a homeotropic alignment in the LC. This is the conventional approach to achieving homeotropic alignment and is the basis of most organometallic chrome complexes and of lecithin which are commmonly used in the research lab. This approach does generate strong homeotropic alignment, but it depends on getting a very uniform, very thin (ideally a monolayer) coating of the material. This is often difficult to achieve. The stability of these materials is often not ideal, and cells do sometimes exhibit ageing presumably because the alignment layers become detached from the surface and dissolve into the LC. In addition these materials are often ionic and so result in an unwanted increase in the conductivity of the LC.
3. Coat the substrate surface with a polymer that induces homeotropic alignment, for example a suitably modified polyimide material. A disadvantage of these materials is that they require a high temperature (typically about 180° C.) bake to cure them. If a plastic substrate is being used this may not be desirable.
4. Inorganic oxides e.g. SiO can give homeotropic alignment when deposited onto the surface at a controlled angle. The disadvantage of this approach is that the deposition can be difficult to control over large areas and requires vacuum deposition.
It has also been suggested in prior art to use liquid crystal polymer layers for inducing planar or tilted alignment in a liquid crystal display. U.S. Pat. No. 5,262,882 describes an orientation layer for inducing planar orientation in a liquid crystal display, consisting of a polymer network in which a low molar mass liquid crystal material is dispersed. U.S. Pat. No. 5,155,160 discloses a liquid crystalline auxiliary layer for inducing tilted orientation in a liquid crystal display cell, which is formed from an anisotropic gel composition comprising a mesogenic diacrylate and a low molar mass liquid crystal mixture. JP 2000-212310, WO 00/46634 and WO 00/46635 disclose an alignment layer for inducing a preferred pretilt in a liquid crystal medium, which is obtained by photoalignment of a photopolymer or a photopolymer/monomer mixture by photoradiation at an oblique angle or by photoradiation with circularly polarized light.
It was an aim of the present invention to provide an alignment layer that induces improved vertical or homeotropic alignment in a liquid crystal medium, and does not show the drawbacks of alignment layers of prior art as described above.
The inventors of the present invention have found that the above drawbacks can be overcome, and satisfactory homeotropic alignment of a liquid crystal medium can be achieved by using an alignment layer of polymerized liquid crystal material comprising rod-shaped molecules with homeotropic orientation, in particular a layer of homeotropic nematic or homeotropic smectic A liquid crystal polymerized material.
In particular it was found that an alignment layer of homeotropic liquid crystal polymerized material exhibits a particularly high surface anchoring energy and yields strong homeotropic alignment in a liquid crystal medium.
Another aspect of the invention relates to the influence of the alignment force on the steepness of the electrooptical curve of an LCD, in particular of a VA mode LCD. Thus, the inventors have surprisingly found that there is a correlation between the surface anchoring energy of vertically aligned liquid crystals, expressed by the tilt anchoring parameter, and the corresponding steepness of the electrooptical curve in LC displays, especially in VA mode displays. In detail, the steepness of the electrooptical curve was found to increase with decreasing anchoring energy.
Based on this finding it is possible to control the steepness of the electrooptical curve of an LCD, in particular of a VA mode display, by using alignment layers of varying anchoring strength. In practical applications of LCDs it is often desired to reduce the steepness of the electrooptical curve to allow for better grey level differentiation. This can be achieved by using inventive alignment layers which exhibit strong anchoring energy and thus lead to reduced steepness.